How to Create Life (in fiction)

In what way are all fiction writers like Dr. Frankenstein?  Answer:  we're all creating life.  Mary Shelley's famous character is a better metaphor for my purposes than the phrase "playing God," because, like the Transylvanian experimenter, we have models to copy from — all the people we see and meet.

The problem is, as Dr. Frankenstein found out, creating life is difficult.  Some writers are better at giving readers a vivid mental picture of their characters than others.  What are the...